The present invention relates to a substrate coated with a photocatalyst film. More particularly, the invention relates to a substrate having functions such as stain resistance, fog resistance and easy wash property.
Attempts are being made to apply to various articles a technique for environmental clarification in which a thin film of titanium oxide functioning as a photocatalyst is used to decompose harmful substances and a technique for obtaining stain resistance by using the titanium oxide film to decompose organic fouling substances and simultaneously make the surface hydrophilic. In these techniques, it is crucially important for the titanium oxide film to have enhanced photocatalytic activity so as to have practical functions.
Various methods for enhancing photocatalytic activity have been attempted. Examples thereof include a technique in which a titanium oxide film having satisfactory anatase crystallinity is formed, a technique in which a porous or granular titanium oxide film is formed so that the film has an increased surface area, and a technique in which a titanium oxide film is formed which has catalytic activity having enhanced sensitivity not only to ultraviolet but to visible light.
A technique for enhancing photocatalytic activity is being attempted, in which the charge separation of the electrons and holes which have been excited by light irradiation in a titanium oxide film is accelerated and the chance of recombination is reduced. JP-A-63-100042 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses the addition of platinum, rhodium or palladium to titanium oxide for the acceleration of charge separation in a titanium oxide film.
An attempt is being made to heighten the photocatalytic functions of a titanium oxide film by adding other elements to the film and thereby attaining valence control and enhancing photoadsorption/photodesorption function. JP-A-10-666879 discloses a technique in which nickel, copper, tin or the like is added to a titanium oxide film to thereby heighten photocatalytic functions, and Japanese Patent Application No. 10-279058 discloses a technique of heightening photocatalytic functions by adding a metal such as magnesium, vanadium, chromium, manganese or molybdenum.
Furthermore, an article having a multilayered photocatalyst film comprising two or more layers superposed in decreasing order of energy band gap (hereinafter referred to as xe2x80x9cband gapxe2x80x9d), which influences the relationship between photocatalytic activity and incident light, from the incident-light side is disclosed in, e.g., JP-A-60-118239 and JP-A-62-68547. JP-A-11-10006 discloses a multilayered photocatalyst film constitution which contains a conductive interlayer of tin oxide formed between a substrate and a photocatalyst film.
However, of the conventional techniques described above, the photocatalyst film which is an even titanium oxide film containing a metal such as a noble metal has the following drawbacks. Charge separation (generation of electron-hole pairs) occurs at the interface between the titanium oxide film as a matrix and the metal. However, when the inner structure of the film is viewed microscopically, there is a high probability that charge pairs recombine before they reach the film surface to effectively perform their photocatalytic functions. Moreover, electron-hole recombination on the titanium oxide film surface is not inhibited. Consequently, this conventional technique has a problem that the attainable photocatalytic activity is not so high.
The photocatalyst film comprising thin semiconductor films superposed in decreasing order of band gap from the incident-light side, in the conventional techniques described above, has an advantage that it causes charge separation in a wider range of incident-light wavelengths and thereby generates more electron-hole pairs. However, this photocatalyst film has a problem that it is difficult to inhibit the electron-hole pairs from recombining in the photocatalyst film and photocatalytic activity cannot always be enhanced effectively.
Furthermore, with respect to the multilayered photocatalyst film constitution containing a conductive interlayer of tin oxide formed between a substrate and a photocatalyst film, in the conventional techniques described above, there is a description in the reference to the effect that the photocatalyst film retains a low charge density and comes to have an increased charge density upon irradiation with intense light, whereby the probability of recombination in the energy band present on the catalyst film surface can be kept low to thereby improve photocatalytic activity. However, this multilayered constitution has a problem that it is necessary to increase addition amount or film thickness for obtaining a certain degree of conductivity, resulting in an increased cost.
Accordingly, an object of the present invention is to provide an article having photocatalytic activity which comprises a substrate, a first n-type semiconductor film as a primer layer formed over a surface of the substrate, and a photocatalyst film made of an n-type semiconductor and formed on the primer layer, wherein the first n-type semiconductor film as the primer layer has an energy band gap larger than that of the photocatalyst film.
When a photocatalyst film is irradiated with ultraviolet ray, electron-hole pairs generate in the film. Of those electron-hole pairs, ones which are present on or have moved to the film surface contribute to photocatalyticactivity. However, when the electron-hole pairs which have generated recombine in an inner part of the film, they no longer contribute to photocatalytic activity. In the present invention, since the photocatalyst film has n-type semiconductive properties, the film has an inflected energy level band structure near the film surface and, because of this, holes are sent preferentially to the surface.
If the thickness of a photocatalyst film is increased in order to enhance catalytic activity, this results in increased chances of electron/hole recombination in inner parts of the film and the band inflection near the film surface becomes relatively small. Because of this, it is difficult to effectively enhance photocatalytic activity by increasing the film thickness beyond a certain level.
In the present invention, an n-type semiconductor film is employed as a photocatalyst film and is bonded to a primer layer which is an n-type semiconductor film having a band gap larger than that of the photocatalyst film. In the multilayer structure of the present invention, the band structure is inflected so that the two films have the same Fermi level. Furthermore, the Fermi level in the photocatalyst film and that in the n-type semiconductor film as a primer layer are located just below the conduction band. Since the band gap in the primer layer is larger than that in the photocatalyst film, the upper edge of the valence band in the primer layer is located below the upper edge of the valence band in the photocatalyst film. Because of this, the holes generated in the photocatalyst film according to the present invention travel so as to recede from the primer film, i.e., travel toward the surface of the photocatalyst film.
In the photocatalyst film according to the present invention, since holes travel preferentially to the film surface, the proportion of holes which recombine with electrons in an inner part of the film and thus disappear without contributing to photocatalytic functions can be reduced for the reasons described above. Consequently, photocatalytic functions can be improved effectively according to the film thickness.
The photocatalytic functions in the present invention are mainly attributable to the presence of holes near the surface of the photocatalyst film. The film is hence highly active in oxidation reactions. For example, the photocatalyst film has been improved so as to have practically useful effects on, e.g., the decomposition of formaldehyde, decomposition of volatile organic compounds (VOC) causative of offensive odors, and decomposition of organic fouling substances which impair the hydrophilicity of glass surfaces.